Sound-transmission, sound-recording, and sound-reproducing system



p 21, 1937- A. D. BLUMLEIN 2,093,540

SOUND TRANSMISSION, SOUND RECORDING, AND SOUND REPRODUCING SYSTEM Filed Dec. 13, 1952 2 sheets-sheep 1 .fi i f i v w l a1 i a; 62 nun H,

1* p l Y I Fig.2.

Sept- 1937- v A. D. BLUMLEIN 2,093,540

SOUND TRANSMISSION, SOUND RECORDING, AND SOUND REPRODUCING SYSTEM Filed Dec. 15, 1952 2 Sheets-Sheet 2.

Patented Sept. 21,? 1937 umrsn STATS zseisn PATENT rice scum) TRANSMISSION, SOUND RECORD- nrc, AND sonnn nsrnonuonve srs- TEM Alan Dower Blumlein, London, England, assignor to Electric and Musical Industries, Limited,

Hayes, England Application December 13, 1932, Serial No. 647,057

In Great Britain December 14, 1931 23 Claims.

This invention relates to the transmission, recording andreproduction of sound and is more particularly directed to systems for recording and reproducing speech, music and other sound 5 effects. It is applicable in particular, although not exclusively, to systems associated with picture effects as in talking motion pictures.

The fundamental object of the invention is to provide a sound recording, reproducing and/or transmission system whereby there is1,conveyed to the listener a realistic impression that the intelligence is being communicated to him over two illusion that the sound is coming, and is only coming, from, the artist or other sound source presented to the eye.

The invention is not, however, limited to use in connection with picture effects, but may, for example, be used for imp-roving the qualities of public address, telephone or radio transmission systems, or for improving the quality of sound recordings. When recording music, considerable trouble is experienced with the unpleasant effects produced by echoes which in the normal way would not be noticed by anyone listening in the room in which the performance, is taking place. An observer in the room is listening with two ears, so that, echoes reach him with the directional significance which he associates with the music performed in such .a room. He therefore discounts these echoes and psychologically focuses sound sources, sound waves emitted thereby, and

the human ears will be briefly summarized.

function is well known and has been employed to considerable extent for example in subaqueous directionaldetection in which two microphones are connected by headphones, one to each ear of an observer, the two channels between the microphones and the two ears being kept entirely separate.

With two microphones correctly spaced and the two channels kept entirely separate, e. gsby using headphones, it is known that this directional effect can also be obtained, for example, in a studio. If, however, the channels are not kept separate (as, for example, is the case in previously proposed arrangements for recording and/or reproducing sound, in which sounds picked up by a plurality of microphonesare led to loud speakers which take the place of the headphones) the effect is almost entirely lost and such systems have therefore not come into common use since they are quite unsatisfactory for the purpose. The present invention contemplates controlling the soundyemitted for example by such loud speakers, in such a way that the directional effect will be retained.

The operation of the ears in determining the direction of a sound source is not yet fully known but it is fairly well established that the main factors having effect are phase differences and intensity differences between the sounds reaching the depending upon the frequency of the sounds emitted. For low frequency sound waves, there is little or no difference in intensity at thetwo ears but there is a marked phase difference. For a given obliquity of sound, the phase difference is approximately proportional to frequency, representing a fixed time delay between sound arriving at the two cars, by noting which the brain decides the direction from which the sound arrives. This operation holds for all frequencies up to that at which there is a phase difference of 7r radians or more between sounds arriving at the two cars from a source located on the line joining them; but above such afrequency, if phase difference were the sole feature relied upon for directional location, there would be ambiguity in the apparent position of the source. At that stage, however, the head begins to become effective as ,a baiile and causesnoticeable intensity differences between the sounds reaching the two ears, and it is by noting such intensity difierences that the brain determines direction of sounds at higher frequencies. It has been stated that the fre-. quency at which the brain changes over from phaseto intensity-discrimination occurs at two ears, the influence which eachof these has I about 709 cycles per second but it must be understood that this may vary within quite wide limits in different circumstances and from person to person, and that in any case the transference is not sudden or discontinuous but there is considerable overlap of the two phenomena, so that, over a considerable frequency range, differences of both phase and intensity will to some extent have an effect in determining the sense of direction experienced.

From the above considerations, it will be clear that a directional effect is to be obtained by providing impressions at the two cars of low-frequency phase differences and high'frequency intensity differences, and it would appear that in reproducing from two loud speakers the diffs..- ences received by two microphones suitably spaced to represent human cars would give this effect to a listener if each microphone were connected only to one loud speaker. It can be shown, however, that phase differences necessary at the ears for low-frequency directional sensation are not produced solely by phase differences at two loud speakers (both of which communicate with both ears) but that intensity differences at the speakers are necessary, while initial intensity differences from the sources necessary for hi.i.1-fre-- quencies are not sufficiently marked when the sounds reach the ears, and to produce suitable effects, therefore, the initial intensity differences must be amplified. It is for this reason that the aforementioned methods previously proposed are not successful in achieving the desired effect,

'these necessary alterations not in those arrangements having been not understood or in any way attained.

It will be seen, therefore, that the invention consists broadly in so controlling the intensities of sound to be, or being, emitted by a plurality of loud speakers or similar sound sources, in suitable spaced relationship to the listener, that the listeners ears will note low-frequency phase differences and high-frequency intensity differences suitable for conveying to the brain a desired sense of direction of the sound origin. In other words, the direction from which the sound arrives at the microphones determines the characteristics (more especially, as will become apparent hereinafter, the intensities) of the sounds emitted by the loud speakers in such a way as to provide this directional sensation.

It must be understood that the manual control by an observer of intensities of a plurality of loud speakers spaced round a motion picture screen has previously been proposed but this method suffers considerably from the defects indicated above, and in any case is very difiicult and inconvenient to operate. No novelty for mere intensity control per se is, however, claimed, except insofar as the nature of the control is such. as to provide the necessary relative phase and intensity difference sensations.

-If, in accordance With the invention, the sound is first recorded and subsequently reproduced from the records, the control may be wholly effected either during the recording or during reproduction, or may be partially carried out in each stage.

More specifically, the invention consists in a system of sound transmission, recording and/or reproduction wherein the sound is picked up by a plurality of microphone elements and reproduced by a plurality of loud speakers, comprising two or more directionally sensitive microphones and/or an arrangement of elements in the transmission circuit or circuits whereby the relative loudness of the loud speakers is made dependent upon the direction from which the sounds arrive at the microphones.

The invention also consists in a system of sound transmission, recording and/or reproduction wherein the sound is received by two or more microphones and wherein, at low frequencies, difference in the phase of sound pressure at the microphones is reproduced as difference in volume at the loud speakers.

The invention further consists in a system of sound transmission, recording and/or reproducticn, in which the original sound is detected by two or more microphones of a type such as velocity microphones whose sensitivity varies with the direction of incident sound, and in which the dependence of the relative responses of the microphones to the direction of an incident sound Wave is used to control the relative volumes of sound emitted by two or more loud speakers.

The invention also consists in a system of sound transmission, recording and/or reproduction wherein impulses from two microphones transmitted over individual channels are adapted to interact whereby the two impulses further transmitted consist in half the sum and half the difference, respectively, of the original impulses, said impulses being thereafter modified to control the relati'e loudness of loud speakers whereby the sound is to be reproduced.

The invention also consists in a system of sound transmission, recording and/or reproduction wherein the sound is picked up by two directionally sensitive microphones which are so spaced and/ or with their axes of maximum sensitivity so directed relative to one another and to the sound source, that the relative loudness of loud speakers which reproduce the impulses is controlled by the direction from which the sound reaches the microphones.

The invention also consists in a system as set forth above wherein the impulses are recorded in the transmission between the microphones and the loud speakers.

The invention also consists in a system, as set forth above, wherein two impulses are mechanically recorded in the same groove.

The invention also consists in a system, as set forth above, wherein the impulses are transmitted by radio telephony.

The invention also consists in a system, as set forth above, in combination with means for the photographic recording or transmission and/or reproduction of pictures.

The word channel, as employed herein, means an electric circuit carrying a curren, having a definite form depending upon the original sounds in the studio. Thus, two channels may be different not because the average intensities or types of current in them differ but because they originate from two microphones in different positions in the studio.

Further features of the invention will become apparent from the following description of various methods and modes of carrying it into effect but it must be understood that the different forms described are given merely by way of example and do not impose any restrictions upon the scope of the invention or the manner and means whereby it may be accomplished.

The description will be more readily understood by reference to the accompanying drawings, wherein Figure 1 represents diagrammatically the assembly of one system according to the invention,

Figure 2 represents a microphonic arrangement for use according to one form of the invention,

Figure 3 represents atransformer arrangement employed in one form of the invention, and

Figure 4 shows a symbolic representation of th arrangement shownin Figure 3,

Figures 5, 6 and 7 represent variouscircuit arrangements applicable to various forms of the invention, while Figures 8, 9, 10 and 11 represent different forms of sound recorders which may be employed.

It will be clear that the invention is particularly applicable to talking motion pictures and the following description will therefore be given with reference to this application. In one form of the invention convenient for this purpose and shown in Figure 1, the sounds to be recorded and reproduced with the pictures may be received from a source a by two pressure microphones a1. a2 mounted on opposite sidesof a block of wood or baffle b which serves to provide the high fre quency intensity differences at the microphones in the same way as the human head operates upon the ears as indicated above. The outputs from the two microphones are, after separate amplification by separate similar amplifiers bi,

b2, taken to suitably arranged circuits 0 comprising transformers or bridge or network circuits which convert the two primary channels into two secondary channels which may be called the surnmation and difference channels. These are arranged so that the current flowing into the summation channel will represent half the sum, or the mean, of the currents flowing in the two original channels, while the current flowing into the difference channel will represent half the difference of the currents in the original channels.

One convenient transformer arrangement for this purpose is shown in Figure 3 wherein input currents from amplifiers b1, b2 are separately fed each to two primary windings, one on each of two transformers, the secondary winding of {each transformer providing a sum or difference output current on account of the senses in which the pimary coils are woundas shown. A diagrammatic representation of a sum and difference arrangement (which may consist of a transformer similar to that of Figure 3 or any other suitable arrangement of circuit elements) is shown in Figure 4. i

' In accordance with the form of the invention being described, the two outputs from the sum and difference'arrangement are modified in order to obtain subsequently the desired sound effects and one convenient circuit arrangement for effecting this is shown in. Figure 5 which represents the portion of the'circuits indicated bye in Figure l 1. Assuming the original currents differ in phase only, the current in the difference channel will be role 75 the voltage across the condenser f in the difference channel. These two voltages are then combinedand reseparated by a sum and difference process such as previously adopted so as to produce two final channels. The voltage in the first final channel will be the sum of these voltages and the-voltage in the second final channel will be the difference between these voltages. Since these voltages were in phase, the two final channels will be in phase but will differ in magnitude. By choosing the value of the shunt resistance 2' in the summation channel and the shunt condenser fin the difference channel for a given frequency, any degree of amplitude difference in the final channels can be obtained for a given phase difference-in the original channels. For the low frequencies, it can be shown that the phase difference between the waves will, for a given obliquity "in-the difference circuit will have the effect of producing a fixed intensity difference in the final channels for a given obliquity at all low frequencies.

For the higher frequencies, as indicated above, it is not necessary to convert phase shifts into amplitude differences, but simply to modify amplitude differences. The shunt condenser f in the difference circuit is therefore built out with a resistance k whose value determines the degree of modification.

In building this circuit, the capacity of the condenser is of such value that its impedance issmall compared with that of the series resistances d and 6 over the whole working range, wlnle the value of resistance is such that it equals the reactance of the condenser at approximately the frequency above which it is desired not to convert phase differences into amplitude differences. that of i, in. which casethe amplitude differences for high frequencies are augmented in the final outputs. I

It may be found necessary to employ more complex circuits than the shunt resistance 70 and condenser j in the summation circuit and shunt resistance 2' in the difference circuit, which, however, form the basic arrangement. However, it must be understood that the circuits employed may be considerably modified as required without departing from the scope of the invention.

The outputs from the modifying circuit 0 (Figure 1) are passed to amplifiers d1, (Z2 and thence to loud speakers e1, e2 suitably disposed on each side of a picture screen. It is to be understood that Figure 1 merelytraces the passage of intelligence from the source a to a recipient and no recording or reproducing system has been shown. Such may, however, be inserted anywhere along the electrical circuit such for example as between amplifiers b1, b2 and modifying assembly 0, or between assembly c and amplifiers d1, d2.

In the latter case, the impulses transmitted through the two channels as indicated above may, for example, be recorded on two sound tracks on a film by any suitable or known means, each of which records may comprise either a sound track of constant density and variable width (e. g. an oscillograph record), or a sound track of constant width and variable density (e. g. a light valve record).

From the above description, it will be clear that obliquity of the direction of sound wave propaga- The value of k may be greater than tion relative to the microphones a l, (12 will produce differences of intensity at the loud speakers so as to give an impression to an observer of oblique sound incidence.

If two very small microphones are used and placed very close together it may be found possible to obtain microphone outputs which do not differ appreciably in amplitude but only in phase for all working frequencies. In this case, the modifying circuit may be arranged to convert phase differences into amplitude differences throughout the entire frequency range. The phase differences dealt with at the low-frequencies, however, may be so small that, in this case, slight differences in the two microphone circuits would have very large effects. On this account, microphone spacing of the same order as that of the human ears is most suitable.

It will be appreciated that the amount of modification necessary to the impulses transmitted through the summation and difference channels as indicated above depends upon a number of factors, including the relative spacing of the microphones and of the loud speakers, and the size and positioning of the screen. It can be shown that for low frequencies w, the degree of modification required in the difference channel as compared with the modification in the summation channel is given by:-

where v=velocity of sound.

y fraction of half picture film width which the image of the sound source is off centre.

0=angle of obliquity, in radians, of the source from the median plane between the microphones.

k=effective distance apart of the microphones.

s=width of screen of theatre.

r:distance apart of loud speakers in theatre.

This expression in effect gives the impedance of the shunt capacity I in the difference channel in terms of the resistance 2' in the summation channel. It holds for all frequencies where k is small compared to the wavelength, and is based on the assumption that the 0 is small and that :1: and s are small compared with the distance of the listener from the screen and loud speakers.

The portion is a factor of the recording, and is constant for a given arrangement if either the camera is in line with the microphones and the centre of the picture, or the action does not move appreciably to or from the microphones and camera. When recording, the relative distances of camera and microphones and the focal length of the lens may be adjusted to maintain this factor a constant.

The expression is a constant for the theatre. As regards low frequencies only, the distance apart of the speakers need not exceed the screen width, but should certainly not be closer than per cent. of the screen width. The closer the loud speakers, the greater the necessary power handling capacity, but the less the troubles introduced by formation of stationary waves.

For the high frequencies, no definite expression can easily be obtained, and the modification used will probably have to be gauged empirically by trial and error.

The arguments and formula given above are based on a direct wave analysis and may have to be considerably modified in order to allow for refiection of other acoustic eifects. It is preferred, therefore, to introduce the modifications it is proposed to employ at the theatre, since all factors will then enter into considertion. It will be clear that, as indicated above, the modifying networks and channel arrangements may be employed between the microphones and the film during recording, or between the record and the loud speakers during reproduction, and the latter course, in addition to allowing of adjustment of the arrangements to suit the particular theatre as indicated above, has the additional advantage that the sound film can be reproduced by a single reproducing head or channel if, for example, one of the dual arrangements breaks down, or in a theatre which, having one installation, does not wish to go to the expense of installinga second apparatus.

In order to employ successfully a system of the kind described above, it is necessary to carry out preliminary experiments to determine the most suitable value of modification to be employed for each recording, and it is also necessary to standardize various factors entering into every recording. In the preliminary experiments, before recording, volume indicator measurements may be made with a standard sound source placed at the extremes of the set, i. e., the space within which recording is to be effected, and from these the proposed modifying network laid out. A further experiment may also be effected to standardize phase angles on the film. At the theatre, a simple adjustment may be provided to check and balance the input to the two channels, a length of test film being used for this purpose. It will thus be seen that the total theatre equipment necessary is very simple and consists in a transmission modifier (comprising two or four transformers, for example, artificial line resistances and the control network, which may be no more than a condenser and a resistance) and two normal soundreproducing heads or pick-ups, or one especially designed head or pick-up adapted to separate the two recordings to two complete reproducing channels. There is no reason why the second channel used should not be the stand-by channel now often installed for safety since if, as indicated above, one of the channels breaks down, reproduction may be continued without serious consequences on the other channel only.

In connection with the standardization indicated above, while the binaural transfer frequency (from phaseto intensity-discrimination) need have no definite significance in recording, since it is a function of the human. brain, it is, nevertheless, necessary to fix a change-over frequency from highto low-frequency working for recording, since this frequency fixes the values of the elements in the modifier and thus the form of modification to be used, the distance apart of the microphones and the form of bafiie between them. Any convenient frequency may be chosen as standard after experience has decided which is most suitable. Instead of stand ardizing, it may be possible from the preliminary experiments to allow electrically for variation of microphone positions and/or of microphone spacing (although the latter would be extremely difi'icult) and it must be understood that this arrangement falls within the scope of the invention.

The above analysis is based upon considerations which take no account of sound reflections or interference during reproduction. The reflected sound waves which arise during recording will be reproduced with a directional sense and will sound more natural than they would with a non-directional reproducing system. If

, difficulties arise in reproduction they may be overcome by employing a second pair of loud speakers differently spaced and having a different modifying network from the first pair; or a I obliquity of the sound varies only within such very small limits as to avoid ambiguous results) may be employed to give some such effect even high frequencies. The system so far described employs, to receive the sound waves, two non-directional microphones, e. g., pressure microphones.

Directionally sensitive miorophonesmay also be employed spaced a small distance apart, the outputs being modified as indicated so that the relative outputs ofthe loud speakers are controlled both by differences in phase and differences in magnitude of the microphone outputs. Such directionally sensitive microphones may be,

but are not necessarily, of the type known as velocity microphones.

Velocity or moving conductor microphones (e. g., moving strip microphones) are very suitable for any system according to the invention and, in addition to use with circuit arrangements described above, they may also be employed with various alterations in the circuits. These microphones givea response varying as the cosine of the, angle of incidence of the sound relative to the direction of normal or optimum incidence, and they therefore have the advantage that a specific degree of loud speaker output separation may be obtained without phase-conversion or like network modifications.

Three general arrangements employing velocity microphones are possible, and in all cases the microphones are placed as-near together as possible instead of being spaced as artificial ears, as in the case of pressure microphones.

(1) Two velocity microphones are placed one with its axis directly facing in the direction of the centre of the scene, and the other with its axis at right angles to that direction. A performer speaking from the middle of the scene perienced between them and if their outputs are summed and differenced after a suitable amount of relative amplification the two final channels differ in magnitude in the correct manner for operating the loud speakers to give the desired directional effect. Such sum and difference arrangement differs from the modifying network employed with pressure microphones in that the pressure type provide phase differences (whereby direction is determined) which have to be converted, whereas with the velocity type the edgeon microphone provides an output proportional to the obliquity of the source. A suitable modifying arrangement for this form of the invention is shown in Figure 6. This is substantially identical with that shown in Figure except that the shunt condenser f and resistance is in series, and the shunt resistance 2 are replaced by shunt resistances lm which are preferably variable as shown. These lines therefore form artificial attenuators and, by altering their relative attenuation, the intensity difierences in the two lines corresponding to a given obliquity of sound is controlled.

(2) Two velocity microphones, or microphone elements, may be placed with their axes perpendicular to one another and each axis at 45 to the direction of the centre of the screen. This arrangement is represented diagrammatically in Figure 2 wherein n and 0 represent two velocity, or directionally sensitive, microphones one above the other arranged perpendicular to one another and at equal angles of 45 to the direction of the centre of the field from which sound is to be received. It will be clear that movement of the sound source w laterally to a position p removed from the centre of the field will result in the sound waves striking 0 at a more acute angle than they strike n and differences in the microphone outputs will result. The microphones are sufficiently close together to render phase differences of the incident sound negligible and the output amplitudes therefore differ approximately proportionally .to the obliquity of the incident sound. They may, therefore, be amplified simi larly and supplied directly to the loud speakers to which they Will give the correct amplitude differences for the desired directional effect, provided the relationship between the various dimensions of the recording and reproducing lay-outs are correct. If it is desired to accommodate any differences between the lay-outs, the outputs may be modified by networks, in the manner described, suitably to increase or decrease the differences between them. An arrangement such as shown in Figure 6 is suitable for this purpose, and such an arrangement may, of course, also be employed even if the lay-out is correct if it is desired for any reason to control or modify the amplitude differences of the loud speaker outputs.

(3)Two microphones may be arranged with the two axes lying symmetrically to the direction of the centre of the field and with an angle between them of say 0 degrees, so that sound from a performer at the centre subtends an angle of degrees to each microphone. If 0 is small, a small movement of the performer to one side is sufficient to make one microphone edge-on and to reduce its output to zero, while if 0 is large a large movement of the performer is necessary to do this. By making 0 adjustable, different layouts may be accommodated without the modifi cation indicated under (2), and it will be clear, also, that this provides a method of directional sound transmitting, recording and reproduction which avoids the necessity of combining and reseparating the two channels.

The microphone elements in any of the above cases may be enclosed in a single casing if .de-

sired for convenience, and may also be positioned in a single magnetic system common to both.

Two velocity microphones set with their axes symmetrically inclined to the direction of the centre line of the scene, may, if placed one above the other, be employed also to provide significance of vertical as well as horizontal movement of the sound source. Such vertical displacement of the source will, in this arrangement, give phase differences to the outputs, while lateral displacements give amplitude differences, and these can be separated, the phase differences converted to intensity differences by modifying networks, as described, and the resulting impulses employed to operate four or more loud speakers distributed around the screen, the transmission occupying, however, only two channels. A similar effect may be obtained with a plurality of pressure microphones by employing suitable modification previous to transmission.

In obtaining a complete directional sound picture, i. e., both horizontal and vertical directional effects, the invention is not limited solely to the use of two microphones. A plurality may be employed and their outputs suitably collected and/or modified and/or separated to transmit suitable differences of impulses to a plurality of loud speakers. The general feature is that two transmitting channels receiving impulses from two or more microphones, for example, communicate two directional senses at right angles to one another, the sounds whereby this is done being provided by a plurality of loud speakers.

It will be seen that, while with pressure microphones it is preferred to transmit phase diferences rather than amplitude differences and convert from one to the other in the last stage, with velocity microphones it is more convenient to transmit the two channels in phase but at different amplitudes, the only modification then necessary being an increase or decrease of the amplitude differences should the reproducing lay-out differ from the recording lay-out or should more than two loud speaker positions be used.

There is a simple method by which modifications for increase or decrease of differences between channels may be effected if no conversion of phase differences into amplitude differences is required. The methed is particularly useful for the operation of more than two loud speakers, and is also useful for working into high impedances such as the grid impedance of a thermionic valve. The arrangement is shown diagrammatically in Figure 7 If the transmission is effected in the form of two channels T8 of similar phase but different amplitudes, an alteration of these amplitude differences may be effected by connecting one wire of each channel 1" and s together at t and connecting a choke it between the other two wires of the two channels. The outgoing channels 1) and w, whose difference is to be a modification of the original difference, are connected by one wire each to the common point t of the original channels, and by their other wires to tappings along the choke u, If the differences are to be increased, the tappings at which the output channels are connected lie outside the tappings to which the input channels are connected, so that the choke operates in effect as an autotransformer amplifying the difference voltages. Similarly, for a reduction of differences, the output channels are tapped intermediately between th two input channels. Modifications of this arrangement in which the devices are balanced about earth, etc. may be arranged, but the chief advantage is that the modification is varied entirely by altering tappings along a transformer or choke, and that no great power loss is involved.

This arrangement of a choke or transformer is well suited to working a number of loud speakers for binaural reproduction. In this case, the two outputs from power valves are fitted to a choke such as 11. along which the loud speakers are tapped. The position of the loud speaker tappings can be adjusted to suit their relative positions, and it can be arranged that the valves are working into their best impedances. Transformers may be used to ensure the speakers taking their correct fraction of the output.

While, in connection with the above described systems, it is suggested that when it is desired to record the sounds for subsequent reproduction this may be done upon a film, the invention is not limited to that medium since the recording may, if desired, be effected on discs or cylinders of suitable material. In carrying out the invention in this manner, the two channels may, if desired, be recorded in separate grooves but it is preferred that they be recorded in the same groove having a hill-and-dale and also a lateral cut movement. For the purposes of television, previous proposals have been made whereby a wax disc has a sound record as a hill-and-dale cut and a picture record as a laterally cut V-shaped groove at the bottom of the hill-and-dale groove, or vice versa. Such records appear unsuited for separate and distinct sound recordings since undoubtedly considerable cross-talk between the two recordings would occur. They can, however, be used for two channels of the kind contemplated in the present invention, one being only slightly different from the other, since a certain amount of cross-talk in this case does not matter, or can be allowed for. Furthermore, the records now proposed are distinguished from those previously known in that both channels may separately be recorded in one groove and may be recorded by a single recording tool (either of moving iron or moving coil type) and be reproduced therefrom by a single reproducing device or pick-up.

If the two channels being recorded are directly picked up from two microphones, or are intended to work unmodified into two speakers, that is, with intensities .and qualities similar, it is preferred not to cut one track as lateral cut and the other as hill-and-dale, but to cut them as two tracks whose movement axes lie at 45 to the wax surface, or at some other convenient angle dependent on the relative available intensities from lateral cut and hill-and-dale respectively. If, however, the two channels recorded are such as summation .and difference channels, it is preferred to separate them completely into pure hill-anddale and pure lateral out, i. e., to make the recording axes normal and tangential to the wax surface.

The result in the two above suggested cases is Very similar since channels recorded at 45 to the wax surface give their sum and difference as the effective lateral and hill-and-dale amplitudes.

It will be appreciated that a record cut as a combined hill-and-dale and lateral cut record may be reproduced, if desired, as two skew direction cuts, the basic principle being that the groove has amplitude in any direction in the plane at right angles to the direction of wax movement, and the recording and reproducing directions may be chosen as any pair of axes lines, not necessarily at right angles, in this plane.

It would appear that, for such a record, a material other than that now used for lateral cut H vertical.

records would be desirabla and a material of the nature of cellulose acetate isindicated.

The track section is preferably adapted to work with a sapphire and have a sufficiently fine angle to give lateral as well as vertical control tothe to thewax surface.

Figure 8 shows schematically a recorder of this kind suitable for producing records having complex cuts. l and 2 represent the driving elements of two; recorders normally adapted for cutting lateralcut records; These driving elements drive arms 3 and 4 about axes at right anglesto theplaneof thepaper within I and 2.

The ends of these arms are connected by ligaments 5 and 6 to the end of a reed l which extends backwards along an axis perpendicular to'the" paper to supports not shown. This reed carries a cutting sapphire 8. Movements of the recording arms 3 and 4 produce movements in the end of the reed l. Thus, currentsinthe driving element i will cause the reed l to move along an. axis approximately 45 to the vertical rising from left to rightacross the figure; Similarly, currents in the driving element 2 will produce movement of the reed I in an axis at right angles to the former axis, while currents in both driving elements will, of course, result in vertical movement of the reed.

Another such form of recorder shown in Figure 9, represented a moving iron recorder, may

consist in a short reed 9 mounted closely above T and parallel with the wax track and carrying the cutting sapphire 8. This reed 9 may extend backwards perpendicularly to the paper to supports (not shown) which jointhe top of a laminated pole system l0 tofComplete a polarizingmagnetic system therewith. The two laminated arms of the pole piece It! extend down towards the free endof thereed 9. These arms form two-poles in a direction at to the wax surface.

adjacent to a square portion of the reed at its freeend, each pole being adapted to pull the reed reed maybesuitably damped, e. g., by a rubber line, and have a resonant frequency at the top'of,

or above, the working range. The two pole pieces may be wound with speech coils and the energization of one of these moves the sapphire in an upward direction at 45 to the wax surface. The terminals of one channel are connected to main winding I2 and compensating windingll. The

terminals [6 of the other channel are connected to main winding M and compensating winding [3.

Current in either channel will pdllthe reed towards the pole carrying the main winding, the

purpose of the compensating winding being toprevent movement of the reed away from the other poledueto the flux drawn away from this poleby the main winding. With the winding shown, currents in either channel will cause the reed to cut a track at approximately 45 to the By a suitable re-arrangement of windings, or by a suitable transformer connection between the channels and the terminals of the recorder as shown, anyother movement axes may be obtained. a 1

An alternative movingicoil design which may employ electromagnetic damping may consistof a it is moving member in theshape of a T as; shown in Figure 10. The recorder sapphire 8 is supported on a light T member ll, whichis supported at l8 The by elastic means such that it mayrotate about this point, and may also translate vertically, although it is resistant to movements in the plane of the paper. The device is driven by moving coils l9 and 20 which are located in annular gaps in a magnetic system, not shown. Current in one of the moving coils tends to both rotate and translate the device so that the sapphire 8 moves along an axis at approximately 45 to the vertical. The movement of this device may be damped and equalized along the lines described in British patent specification No. 350,998. As before, any required axes of movement may be obtained by suitable interconnection of the two driving coils. Such a movement preferably has the same natural frequency for both rotation and translation. Further, the distributionof mass is preferably such that a small instantaneous force applied at one coil produces no movement at the other.

Figure 11 shows another form of recorder similar in principle to the one shownin Figure 10 except that a moving iron drive is employed. The member I! moving about axis 58 is constructed of magnetic material, or has a magnetic upper portion. The E shaped member 2| is polarized either by being partially permanently magnetized, or having a magnetizing winding on it, so that the central pole is of opposite polarity to the two outer poles. Speech windings on the outer poles are brought out to terminals l5 and it to which the two channels are connected.

In all the devices described above, the angles of the axes defining themovements ofthe sapphire can be altered by suitably connecting the speech windings; for instance, axes which are normally inclined at 45 to the wax surface can be converted into pure hill and dale and lateral cut axes by arranging thatthe speech windings are in series aiding for one channel and opposing for the other channel. In like manner, any

axis conversion can be effected by suitably com-- bining the channels through transformers.

In designing an electric pick-up to reproduce both channels, care must be taken that the inertia is kept as low as possible, and with this in mind, a very light replica of any of the above described recorders may be employed. Preferably, a moving system in the form of a T following the lines of the moving iron recordershown in Figure 11 is employed as best suited for the purpose. Since the fundamental resonant frequency of a pick-up appears to be of no critical importance as regards its characteristic, it may not be necessary to adjust the resonant frequency in the two modes to the same value, which would simplify the design. Adjustments for sensitivity in the two modes may be made by out which represents the sum of the two chan-.

nels to the speakers might have a frequency range extending to 10,000 C. P. S. whereas the hill-anddale cut need transmit frequencies no. higher than 3,000 C. P. S. This would considerably sima.

plifythe design of the recorders and pick -ups in that low inertias would'only be-requirecl for the the phase shift.

lateral cut and design would thus be greatly simplified.

These frequencies are given merely by way of example, and are not necessarily the optimum frequencies for design of this character, which will be determined by other considerations.

In transmitting the two channels indicated in the various systems above described, instead of employing line transmission, radio transmission may, if desired, be employed. Each channel may be separately transmitted or preferably the methods described in the specification of copending Cognate British patent applications Nos. 30,712 of 1931 and 24,423 of 1932, whereby two signals from a common source are transmitted over a single carrier wave, may be employed.

The methods described in those two specifications comprise the separate modulation by signals from a common source of two carrier waves 90 different in phase, which are thereafter combined to form a single wave which is transmitted.

The 90 phase difference enables the wave to be broken into its. appropriate components by suitable means at the receiver and the signals to be separately reproduced. Another method also de'- scribed comprises the modulation of a single carrier wave by two signals from a common source, one modulating the carrier in frequency and the other modulating it in amplitude. Suitable receiving apparatus enables these signals to be separately received and reproduced independently. It should be added that duplex radio-signalling is well known and the novelty thereof in the present application resides only in its use in the binaural systems described above in accordance with the present invention.

In such a system, the secondary impulses obtained as sum and difference effects from the initial impulses may conveniently be obtained by shifting the phase of the transmitted carrier wave through 45. The general transmission arrangement may be substantially as described in the specification referred to and any convenient or known means may be utilized for effecting If, for example, B and B in Figure 2 of that specification form parts of balanced modulator systems, the outputs from which are free from any carrier waves, a separate carrier wave amplifier may be added enabling a carrier wave to be injected into the aerial in any desired phase. Alternatively, transmission may be effected by modulation of phase and amplitude of the carrier wave by the signals originating from the two microphones, and reception effected by apparatus such as shown in Figure 3 of the specification referred to, in which case signals are received proportional to the sum and difference respectively of the signals modulating the carrier wave.

The hereindescribed system, while being especially applicable to talking pictures, is not limited to such use. It may be employed in recording sound quite independently of any picture effects and in this connection (as well as when used in cinematograph work)v it seems probable that the binaural effect introduced will be found to improve the acoustic properties of recording studios and to save any drastic acoustic treatment thereof while providing much more realistic and satisfactory records for reproduction. Furthermore, the system may clearly be employed when the microphone outputs are led directly to the loud speakers instead first of being recorded, and such an arrangement may, for example, be employed in public address systems in which directional sound effects are desired; In general, the invention is applicable in all cases where it is desired to give directional effects to emitted sound. Also, in all cases, both when the impulses are fed directly to the loud speakers and when they are recorded for subsequent reproduction, the total modification and/or interaction of the channels may be accomplished in more than one stage. For example, using pressure microphones, the low frequency phase differences may be amplified, the medium frequency phase differences converted to amplitude differences, and the high frequency amplitude differences augmented in a first stage of modification; the low frequency phase differences may then be converted to amplitude differences in a later stage of modification. One or both of these stages may occur either before or after the sound has been recorded. In this manner, the very small low frequency phase differences are augmented before they are amplified, so avoiding troubles due to small low frequency phase shifts in amplifiers.

Moreover, the various devices employed for carrying the invention into effect must be understood not to be limited to their use with the other devices in the systems also hereindescribed since clearly many parts, such, for example, as the dual track record prepared by a single cutter, and the multi-strip direction-detecting microphone, are clearly of wide use separately of one another. Such uses in binaural systems as herein described fall within the scope of this invention.

It must finally be understood that the invention is not restricted solely to the details of arrangements of the forms of the invention described above since various modifications may be introduced in order to carry the invention into effect under different conditions and requirements which have to be fulfilled without departing in any way from the scope covered thereby.

I claim:

1. A system of sound transmission comprising a plurality of microphone elements, a plurality of loud speakers adapted to receive and reproduce impulses from said microphone elements, and electrical elements in the circuit between said loud speakers and said microphone elements adapted to control the relative loudness of said loud speakers in accordance with the direction of incidence of the sounds on said microphone elements.

2. A system of sound transmission comprising a plurality of microphone elements, a plurality of loud speakers for receiving and reproducing impulses from said microphone elements, and means for controlling the relative loudness of said loud speakers in accordance with the direction from which the sounds arrive at said microphones, said means including electrical connections in the transmission circuit adapted to combine the impulses from the said microphones and reseparate them into a plurality of impulses of different form.

3. A system of sound transmission comprising a plurality of microphone elements, a plurality of loud speakers for receiving and reproducing impulses from said microphone elements, and means for controlling the relative loudness of said loud speakers in accordance with the direction from which the sounds arrive at said microphones, said means including electrical connections in the transmission circuit adapted to combine the impulses from the said microphones and reseparate them into a plurality of impulses 2,093,540 of different form; and electrical elements adapted-.

to modify the characteristics of said different impulses. r

4. A system of sound transmission comprising a plurality of microphone elements, a plurality of loud speakers for receiving and reproducing impulses fromsaid microphone elements, and means for controlling the relative loudness of said loud speakers in accordance with the direction from which the sounds arrive at said microphones, said means including electrical connections in the transmission circuitadapted to combine the impulses from the said microphones and reseparate them into aplurality of impulses of different form, electrical elements adapted to modify the characteristics of said different impulses, and further electric connections adapted to recombine and again reseparate said modified impulses.

5. A system of sound transmission according, to claim 2 comprising electric connections adapted to combine two original impulses and reseparate them into two different impulses consisting respectively in half the sum and half the difference of the original impulses.

6. A system of sound transmission according to claim 2 wherein said electrical connections comprise a plurality of interacting transformer windings for combining the impulses and reseparating them into different impulses.

'7. A system of sound transmission according to claim 3 comprising an attenuator network adapted to modify the characteristics of said impulses.

8. A system of sound transmission according to claim 3 comprising a phase modifying arrangement of elements adapted to alter the characteristics of said impulses.

9. A system of sound transmission according to claim 3 comprising plain shunt elements in the transmission channels of said different impulsesadapted to modify the characteristics of such impulses.

10. A system of soundtransmission according to claim 2 comprising means adapted to convert, over part of the frequency range, phase differences between the original impulses from said microphones intoamplitude differences between the impulses of different form in the resultant channels.

, 11. A system of sound transmission according toclaim 2 comprising means in the resultant channels for modifying the amount of difference between the impulses therein.

12. A system of sound transmission comprising as microphone elements a plurality of strip conductors so light as to move substantially as the surrounding air, positioned longitudinally in line ing in directions equally inclined to the direction of the centre of the field in which the sound source is located. v

15. In combination with a cinematograph system, a system of sound transmission according to claim 1 comprising electrical modifyingelements in the transmission channels of values defined by the formula.

where the symbols have the herein. i

16. A system of sound transmission comprising a plurality of microphone elements, a plurality of electrical channels adapted to receive impulses from said microphone elements, and electrical elements in circuit between said microphone elements and said electrical channels adapted to control the impulses generated by an elementary sound Wave arriving at said microphone elements, whereby the transmitted impulses are maintained in phase, but are rendered dependent in relative amplitude upon the direction from which the sound arrives at said microphone elements.

17. A system of sound transmission comprising a plurality of directionally sensitive microphone elements, and a plurality of electrical channels adapted toreceive impulses from said microphone elements, arranged so that the impulses in a plumeanings defined rality of said channels are combined and reseparated in such a manner that the impulses of the resultant channels, although not similar to those of the original channels, are modifications of them conveying the same directional intelligence in another form.

18. A system of sound transmission comprising a plurality of microphone elements, means for recording impulses generated in said microphone elements by the incident sounds, means for picking up said recorded impulses from said record, aplurality of loud speakers for reproducing said impulses and means adapted to control the relative loudness of said loud speakers in accordance with the direction from which sounds arrive at said microphone elements, wherein said impulses from said microphone elements are recorded unmodified, and said means for controlling the loudness of said loud speakers are incorporated in circuit between said pick-up means and said loud speakers.

19. A system of sound transmission comprising a plurality of microphone elements, means for recording impulses generated in said microphone elements by the incident sounds, means for picking up said recorded impulses from said record, a plurality of loud speakers for reproducing said impulses, means between said microphone elements and said recording means adapted partially to modify impulses from said microphone elements, and means between said pick-up and said loud speakers adapted to eifect a. further stage of modification of said impulses, whereby the relative loudness of said loud speakers is controlled in accordance with the direction from which sounds arrive at said microphone elements.

20. In a system of sound transmission, a plurality of microphone elements, recording means for simultaneously recording different sets of imfrom which sounds arrive at said microphone elements.

21. In a system of sound transmission, a plurality of microphone elements adapted to move in accordance with the velocity component of the air set into vibration by the sounds being transmitted, recording means for simultaneously recording difierent sets of impulses comprising a cutting tool capable of controlled movement in any direction in a plane perpendicular to the direction of movement of the medium it is cutting, means for picking up said recorded impulses from said record, a plurality of loud speakers for reproducing said impulses and electrical elements in the circuit between said loud speakers and said microphone elements adapted to control the relative loudness of said loud speakers in accordance with the direction from which sounds arrive at said microphone elements.

22. In a system of sound transmission, a plurality of microphone elements, recording means comprising a cutting tool capable of movement in two directions, a co-operating magnetic system whereby movements of said cutting tool are controlled, energizing windings on the poles of said magnetic system for receiving impulses to be recorded, and compensating windings on said poles adapted to neutralize the magnetic effects in one pole of impulses received in the energizing windings of the other pole, means for picking up said recorded impulses from said record, a plurality of loud speakers for reproducing said impulses and means adapted to control the relative loudness of said loud speakers in accordance with the direction from which sounds arrive at said microphone elements.

23. A system of sound transmission comprising two microphone elements adapted to move in accordance with the velocity component of the air set into vibration by the sounds being transmitted, arranged with their axes of maximum sensitivity inclined to one another, two loud speakers adapted to receive and reproduce impulses from said microphone elements, four leads, one from each microphone and each loud speaker, being connected together, and the other four leads, one

from each microphone and each loud speaker being connected to tappings on a common choke adapted to modify the impulses from said microphones prior to reproduction by said loud speakers.

ALAN DOWER BLUMLEIN. 

